Experimental determination of the symmetry of the order parameter in YBCO

At present, the symmetry of the order parameter in the high temperature superconductor YBCO is quite controversial. Recent experiments using SQUIDs and Josephson junctions appear to support competing theories, with some experiments supporting a d_x2_–y2 pairing symmetry for the order parameter and others a s-wave pairing symmetry. We note that a number of factors such as trapped flux, magnetic field gradients and SQUID asymmetries could lead such measurements astray. We use a Scanning SQUID Microscope and a time-reversal invariance test to resolve these experimental problems. We find the order parameter in YBCO has a time-reversal invariant d_x2–_y2 symmetric component. We estimate the amplitude of anyimaginary s-wave symmetric component to be less than 4% and anyreal s-wave component to be less than 82%.